Instant messaging (IM) has grown from simple messaging in the 1960's, bulletin board systems of the 1980's, and messaging applications of the 1990's, into the field of unified communications, which provides real-time communications services such as unified messaging (integrated email, voicemail, fax, instant messaging, and presence information), telephony, and video conferencing. Enabling many of these IM features are a number of messaging and presence protocols, such as Instant Messaging and Presence Service (IMPS), Extensible Messaging and Presence Protocol (XMPP), and Session Initiation Protocol (SIP) with its extension SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE), to name a few.
These protocols allow the establishment of an IM session that may include IM, file transfer, voice, and video. Underlying these protocols is the concept of “presence” which indicates a user's willingness, capacity, or ability to communicate. Before one can initiate an IM session with another, one must ascertain the status of the other, e.g., “off-line”, “busy”, or “do not disturb”. Presence information is periodically shared across the network, while IM is event driven, i.e., nothing happens until a user takes action. Both IM and presence can be used to send a variety of types of information.